Field
The present invention relates to a backlight unit, and more particularly to a backlight unit capable of local dimming.
Description of the Related Art
Since a liquid crystal display device is not able to emit light in itself, it requires a backlight unit for supplying light to the liquid crystal. Here, the liquid crystal display device is generally divided into a directly downward type and an edge type.
In the directly downward type, a light source is located below the light emitting surface of the light guide plate. Since the light source is arranged in a 2-dimensional manner, it is easy to perform the local dimming. However, the entire thickness of the liquid crystal display device is very large and a plurality of light sources (LED) are required. Therefore, the directly downward type is not suitable for mobile devices.
Meanwhile, in the edge type, the light source is located on the side of the light emitting surface of the light guide plate. Therefore, the entire thickness is small and it is possible to implement the backlight unit by means of a smaller number of the light sources. However, since the light source is located only on the side of the light emitting surface, it is difficult to perform the local dimming which divides the light emitting surface into a plurality of sections.
The patent document 1 relates to an edge type backlight unit using a light guide plate with an inverse prism structure on the light guide plate. According to the patent document 1, it is possible to form a backlight unit only by a single light guide plate without additional reflective sheet, diffusion sheet, and prism sheet. This enables to maximize the small thickness of the backlight unit, i.e., an advantage of the edge type. Also, since only one light guide plate is used, there are advantages in cost and efficiency by optical loss reduction. Although the edge type backlight unit using the inverse prism structure of the patent document 1 maximize the advantages of an existing edge type backlight unit, it is still difficult to perform the local dimming.
Meanwhile, the patent document 2 and the patent document 3 are disclosed to solve the limit of the local dimming instead of the thickness. The patent document 2 and the patent document 3 propose edge type backlight units capable of local dimming, and thus, focus on low power consumption and image quality improvement by a high contrast ratio. However, the backlight units disclosed in the patent document 2 and the patent document 3 are in common with each other in that the light is emitted to the front surface of the light guide plate by using the reflection pattern formed on the lower portion of the light guide plate. When the LEDs are disposed within the light guide plate, this causes a hot spot on the light guide plate in which each of the LEDs is included. As a result, when a user views the backlight unit, light uniformity is greatly reduced.
In order to solve the hot spot problem, it has been proposed that additional diffusion sheet and prism sheet are disposed on the light guide plate or each backlight unit is manufactured in the form of an assembled tile. However, according to these methods, it is complicated to individually form each of the backlight units and assembly them. Moreover, by using additional diffusion sheet and/or prism sheet, there are problems of increase of the entire thickness, cost increase, and optical loss at the interface between the sheets.